Packing containers of the non-returnable type are used at present for a great number of foodstuff products, e.g. milk, juice, soups, fruit creams and the like. The packing containers, which may be aseptic and filled with goods previously heat-treated or sterilized in some other manner, are manufactured in packing machines,in that prefabricated, laminated plastic and paper material is converted and sealed to liquid-tight packing containers filled with the particular product. In those cases where the packing containers are not wholly filled with the particular contents a certain free volume or heat space, will be present in each packing container which for the sake of the keeping quality is usually filled with an inert gas, e.g. nitrogen gas. A known machine of this type is presented in Swedish patent application no. 82054941 which also describes the method of manufacture of the packing containers. Packing machines of this type are generally also equipped with a cleaning and sterilizing system by means of which the parts coming into contact with the contents can be cleaned and sterilized. The sterilization generally takes place in that a chemical sterilizing agent, e.g. hydrogen peroxide gas, is mixed with air. This circulates within the machine. On the surfaces, which are of a temperature lower than the dew point of the hydrogen peroxide-air mixture, hydrogen peroxide will condesne out in liquid phase. Subsequently hot, sterile air is blow through the system and vaporizes the hydrogen peroxide anew, so that the machine is dried. In this manner a complete killing of any bacteria present in the machine, which otherwise may have a detrimental effect on the keeping quality of the packed product, is assured.
The inert gas, which is used for filling the free volume of the packing containers, is supplied to the packing machine via a filter which prevents bacteria or other contaminants from accompanying the gas into the packing containers. The filter has to be sterilized before each production and ,moreover,it has to be exchanged at regular intervals. After every exchange a sterilization of the filter and the filter chamber is carried out so as to prevent any bacteria, which have accompanied the exchange of the filter, from reaching the inner, sterile space of the packing machine and the packing containers manufactured there. The sterilization of the filter takes place on known machines by means of saturated steam which during a certain period is passed through the filter, so that the filter and filter chamber as well as the discharge ducts are sterilized. Since the steam condenses in the filter chamber and on the filter, it is necessary, after the resterilization, to blow the parts clean with the help of air, which is conducted through the filter for an appropriate period. Steam and/or air may be passed into the machine, but have to be led out of the syste, which makes it necessary to use valves and ducts not only for the supply of steam and/or air but also for making possible the discharging of the same. At the same time use has to be made of valves to prevent any steam and/or air from penetrating into the duct system which connects the filter to the packing machine, and all this gives rise to a complicated system of ducts and valves which is very susceptible to faults. The process of sterilization of the filter too becomes correspondingly complicated, since it comprises repeated opening and closing of a great number of valves in a certain sequence which has to be accurately followed,if the sterility in the duct system as well as in the packing machine is not to be lost, with attendant, timeconsuming resterilization as a consequence.
It is generally desirable, therefore, to find a method for the sterilization of filters which is less complicated and consequently more reliable than the methods known earlier.